Psychologists have discovered that monkeys are unable to grasp complex grammar necessary for human languages.
The researchers, from the Universities of St Andrews and Harvard found that nonhuman primates appear capable of perceiving and remembering basic sequential order, but not hierarchical structure.
The findings, published in the latest issue of the journal Science, provide the clearest example to date of a ‘cognitive bottleneck’ which could help refine our understanding of how human language evolved.
Dr W. Tecumseh Fitch (St Andrews) and Professor Marc D. Hauser’s (Harvard) paper suggests a sharp limit to animals’ capacity to perceive and generate open-ended communication signals and possible restrictions on other domains of thought.
“There is no doubt that human communication is fundamentally different from that of other animals; the key question is, what computational, conceptual, motor and sensory systems have evolved to create this difference?” said Professor Hauser.
Dr Fitch continued: “Grammar involves rules that allow a limited set of sounds to combine in an unlimited number of ways. These rules can be simple, controlling sequential order, or more complex, generating hierarchical structure. For example, 441334476161 might just be a list of numbers, but +44 1334 476161 is interpreted as a phone number with higher order structure (44 is the UK country code, 1334 is the exchange for St. Andrews, etc).
“Similarly, a musical melody is not just a string of notes: the relationships between the notes are crucial. This is why we say the melody is “the same” when transposed to a different key, even though all the notes have changed.”
Previous studies have shown that monkeys can grasp simple sequential rules governing speech sounds. But studies of monkey music perception suggested that they do not perceive the higher- order structure of melodies: a melody transposed up a step is perceived as completely new and different.
In their research, Dr Fitch and Professor Hauser showed that their monkeys were capable of understanding a simple grammar rule at the ‘finite state grammar’ (FSG) level, which involves simple sequencing. FSG-level rules are spontaneously available to both human infants and nonhuman primates.
However, the monkeys failed to grasp a crucial grammatical component of all human languages, the more complex hierarchical structures governed by phrase structure grammar (PSG). An example of such structure is the ‘If…then’ rule where the words ‘if’ and ‘then’ are linked and dependent upon each other, despite being separated by a large number of words.
“A crucial component of the power of language is phrase structure: our sentences are not just lists of words, but they have a complex structure over and above that, which enables us to communicate causal connections, opinions about statements, complex social ideas and many other things that single words, or unstructured strings of words, cannot,” said Dr Fitch.
The research, conducted at the Harvard Primate Cognitive Neuroscience Laboratory, involved playing recorded examples of the two types of grammar to cotton-top tamarins, a New World primate species that has previously demonstrated successful discrimination of linguistic stimuli. Following this initial exposure, the tamarins heard a series of recordings, some of them violating the rules of Fitch and Hauser’s grammars. When the animals perceived such inconsistencies, they tended to look at the speaker playing the sounds, a behaviour used by experimentalists as an indicator of novelty detection in studies involving both animals and infants.
Based on whether the tamarins looked at the speaker, the researchers determined that the animals were able to perceive violations of the simpler FSG grammar but did not take note of infractions of the more complex PSG grammar.
The research suggests that tamarins are only able to comprehend simple sequential associations of sounds, but not more complex structures.
Dr Fitch explained: “Human language is a system which allows us to communicate about anything, with no obvious limits to subject matter, complexity or level of detail. In addition to the food, predators and sex that many other species communicate about, we can talk about quasars, the Roman Empire, morality or mathematics.”
He continued: “This technique can easily be applied to any other species, and we hope that it will quickly be tried with many others. Songbirds, and great apes like chimpanzees or gorillas, will be of particular interest. And of course it will be important to find out when this ability appears in human development. Do newborn babies already have the ability to perceive phrase structure, or is this something that doesn’t mature till 2 yrs or later, when they start producing grammatically complex utterances?”
DR FITCH firstname.lastname@example.org
DR ZUBERBUHLER kz3@st- andrews.ac.uk
PROF HAUSER email@example.com
A JPEG OF A TAMARIN IS AVAILABLE FROM THE PRESS OFFICE – CONTACTS BELOW
· The research is supported by the National Science Foundation, the National Institutes of Health and the McDonnell Foundation.
· The paper ‘Computational Constraints on Syntactic Processing in a Nonhuman Primate’ by W. Tecumseh Fitch and Marc D. Hauser appears in the current edition of ‘Science’.
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